The invention relates to earth fault protection of a frequency converter and particularly to a method that enables limiting the earth fault current of a frequency converter.
A frequency converter is an electric device used for controlling a load. One frequency converter structure comprises a direct voltage intermediate circuit, the voltage of which is generated of supply voltage, such as three-phase voltage of the supply network, with a rectifier. This rectified voltage of the direct voltage intermediate circuit is then switched with an inverter part to the load in such a way that the desired voltage on average is generated in the load. The output voltage of the frequency converter is thus generated of individual direct voltage pulses, the duration of which is modulated for instance by controlling with a current controller.
A phase-to-earth fault in the motor cable of a frequency converter or in the motor causes fault current in an earthed network from the supply side, which may break the frequency converter. Earlier, fault current was indicated by measuring the sum of currents in the motor phases, input phases or intermediate circuit, and when this sum deviated from zero, fault trigger was carried out. The sum of the currents in the motor phases, i.e. output phases, is zero in a normal operating situation, because typically the loads have no separate return conductor, but all current going to the motor returns along supply conductors. There are also methods in which the indication of a fault situation is based on measuring voltages.
The sum of the currents may be measured either by separately measuring the currents to be added together and generating their sum, or by conducting the currents to be added together through a common current transformer. Since, in any case, the phase currents of a motor are usually to be measured for motor control purposes, it is rather common to measure three phase currents separately and to calculate their sum to detect the earth fault situation. However, in cases where it is desirable that the control of a motor be based on measuring only two phase currents, an inexpensive current transformer has often been used for measuring the summation current, whereby the main circuit of the frequency converter is conducted through this current transformer, i.e. either three input phases, two intermediate circuit conductors (dc+ and dc−) or three output phases.
However, measuring the summation current is problematic in view of the dimensioning of the measuring circuit: if, for example, the sum of three output currents is to be measured with a current transformer, the primary circuit of the transformer must be dimensioned for three different coils, each of which must have a conductor of the magnitude of the rated current, although the actual object of measuring, i.e. the summation current, is zero with regard to thermal dimensioning.
The methods based on measuring voltages are appealing specifically for the reason that in case of earth fault, the potential differences are measurable with high-impedance and thus inexpensive circuits. Roughly speaking, the detection methods of an earth fault based on measuring voltages can be divided into two categories: those measuring voltage between the main circuit of the frequency converter (e.g. potential of the mid-point of the intermediate circuit) and the ground potential, and those measuring voltage loss caused by the earth fault current (e.g. from the poles of a filter reactor divided evenly between the dc+ and dc− busbars). An earth fault situation is in both cases detectable from the abnormal behaviour of the voltage.
Further, a difficulty with all known earth fault indication methods is that indication cannot be made very fast. This is because common-mode current (and corresponding potential swings) is related with switch turns of an inverter also in normal situations. The potential of the motor phase typically swings from dc− level to dc+level (and back) within clearly less than a microsecond. Thus, when charging and discharging, the earth capacitances of the motor and motor cable take rather high-amplitude—yet short—current bursts with oscillation. Due to this, the indication must be carried out fairly slowly with filtering, and the trigger limit cannot be set very near zero in the case of a summation current.